Iraq's Sunni Arab vice president denied terror charges against him and vowed to defend himself in a defiant news conference as rival leaders called for urgent talks to resolve a worsening crisis and the US expressed concern.

Just days after US forces left the country and on the eve of the national unity government's first anniversary Wednesday, Iraq's fragile political truce looked as if it was already unraveling.

A warrant was issued for Vice President Tareq al-Hashemi's arrest on Monday, spurring his Iraqiya bloc to say it would boycott cabinet meetings.

And over the weekend, Prime Minister Nuri al-Maliki, a Shiite, called for the sacking of deputy premier Saleh al-Mutlak, a Sunni who has branded the Shiite-led government a "dictatorship."

The White House voiced concern over the developments as US Ambassador James Jeffrey met Iraqi leaders, although Maliki's office ruled out any mediation over the charges against Hashemi.

"I swear to God that I never committed a sin when it comes to Iraqi blood," Hashemi told a news conference in the Kurdish regional capital Arbil.

"I suggest transferring the case to Kurdistan. On this basis, I will be ready to face trial."

He called for representatives of the Arab League to take part in the investigation and any questioning, and said apparent confessions aired on state television linking him to attacks were "false" and "politicized."

In Washington on Tuesday, US Vice President Joe Biden urged Maliki to work with other parties to resolve the crisis.

Biden spoke by phone with Maliki and with Iraqi Council of Representatives Speaker Osama al-Nujaifi "to discuss the current political climate in Baghdad," the White House said in a statement.

Biden "stressed the urgent need for the prime minister and the leaders of the other major blocs to meet and work through their differences together."

Officials issued the warrant for Hashemi's arrest on Monday, after earlier banning him from travelling overseas.

At least 13 of the vice president's bodyguards have been detained in recent weeks, although it was unclear how many remain in custody.

Hashemi's office said only three were arrested, and has complained of "intentional harassment" in the form of blockade of his home by security forces for several weeks, as well as other incidents.

State television has shown footage of what the interior ministry said were confessions by Hashemi's bodyguards to planning and carrying out terror attacks, and receiving funding and support from the vice president.

The Baghdad security operations command issued two statements on Tuesday, saying security forces would carry out the warrant against Hashemi, and claimed an insurgent who developed car bombs had provided details of the vice president's involvement.

White House spokesman Jay Carney said the United States, whose troops completed their withdrawal from Iraq over the weekend, had "expressed our concern regarding these developments."

US officials in Washington also confirmed that the director of the Central Intelligence Agency, David Petraeus, the former US military commander credited with containing sectarian violence in Iraq, had paid a visit to Baghdad in recent days.

But officials said it was a previously scheduled trip to Iraq and Afghanistan and that Petraeus was not engaged in political talks in Baghdad.

Maliki and other leaders have called for talks to resolve the political crisis, but the premier's spokesman told AFP he would not accept any mediation over the charges against Hashemi.

"The prime minister will not compromise the blood of Iraqis, no matter what the price," Ali Mussawi said.

"The justice system should carry out its role. No one should block the work of justice, and we must allow it to complete all stages of the investigation in Hashemi's case."

Maliki has also called for Mutlak, like Hashemi a Sunni Arab and a member of the Iraqiya bloc, to be sacked after Mutlak said the premier was "worse than Saddam Hussein."

Lawmakers are due to consider Maliki's request to fire Mutlak on January 3.

Iraqiya said it would boycott the cabinet to protest against Maliki's "dictatorship," although it has not pulled out of the government.

The premier's call for urgent talks was echoed by parliament speaker Osama al-Nujaifi and Kurdish regional president Massud Barzani.

In November 2010, Barzani hosted a meeting of Iraqi leaders at which the foundations of the national unity government were laid, ending months of deadlock following elections in March that year, with a cabinet eventually named on December 21.

The bloc, which holds 82 of the 325 seats in parliament and controls nine ministerial posts, had earlier said it was suspending its participation in the legislature.

Iraqiya, which garnered most of its support from the Sunni minority and emerged with the most seats in March 2010 elections, was out-maneuvered for the premiership by Maliki who finished second in the polls but subsequently broadened his power base by striking a deal with another faction.

The political party loyal to anti-US Shiite cleric Moqtada al-Sadr, which met Barzani on Tuesday, lamented that the timing of the crisis, after US troops completed their withdrawal from the country early on Sunday, suggested American forces had held the political system together.